1. Field of the Invention
The present invention relates to an apparatus and methods for continuous beam Fourier transform mass spectrometry. In particular, the invention relates to an apparatus and methods for providing a mass spectrum of a continuous beam of ions.
2. Background Art
Mass spectrometry is an analytical tool for identification of chemical structures, determination of mixtures, and quantitative elemental analysis of organic compounds, based on application of the mass spectrometer.
Mass spectrometer is an instrument used for determining the masses of atoms or molecules found in a sample of gas, liquid, or solid. The mass spectrometer was originally developed as a nuclear physics research tool. Today, mass spectrometers are widely used in various types of institutions, laboratories, industries and other related entities to measure and identify minute quantities of various substances.
Several types of mass spectrometer are currently available. A traditional design of a mass spectrometer is based on the combination of electrostatic and magnetic sector fields. FIG. 1 illustrates how such a mass spectrometer works. In an ion source 1, atoms or molecules are ionized by bombarding them with electrons to become electrically charged atoms or molecules, i.e., ions. The ions are then extracted by an electric field (not shown) to form a beam 3 of ions. A slit 2 is used to define the beam. Beam 3 enters an electrostatic energy analyzer 4, where a sector electric field E1 focuses ions onto an intermediate slit 5. The ion 6 that pass through the intermediate slit 5 then pass into a uniform sector magnetic field B presented in region 7. Thus, ions 6 with energy E in region 7 are deflected into a circular path with radius R=mv/qB by the uniform magnetic field until they strike either a photographic or electronic detector 8 at a location proportional to their mass, since the radius of curvature increases with mass. This forms a mass spectrum that allows one to separate ions with the same charge but different masses. For example, ions strike the detector 8 at position 9 would have greater mass than the than ions the strike detector 8 at either positions 10 or 11. Alternatively, ions 6 can be allowed to sweep across a slit (not shown) in front of a detector by scanning the magnetic field or the accelerating potential.
Another form of mass spectrometry is referred to as ion cyclotron resonance. In this case, ions are either found within or are allowed to drift through a uniform magnetic field where they execute cyclotron motion according to xcfx89=qB/m. The ions can be detected by scanning the magnetic field while applying a sinusoidal electric signal to a pair of opposing plates placed on either side of the ion beam or cloud. A signal is generated by use of a tuned circuit to detect the power absorbed by ions that come into resonance with the applied signal. Alternatively, ions can be detected by measuring the image currents generated on a pair of plates placed orthogonally to the plates used to excite the ions.
Fourier transform techniques have been applied to ion cyclotron resonance to provide a Fourier transform ion cyclotron resonance (xe2x80x9cFTICRxe2x80x9d) mass spectrometer. FTICR uses a uniform magnetic field to trap ions to be analyzed and an excitation pulse to excite the ions into coherent motions so that they can be detected. In FTICR, ion formation, ion excitation, and ion detection are done sequentially in time. Such FTICR mass spectrometers thus have a disadvantage of low duty cycle for continuous analyte consumption.
Fourier transform quadrupole mass spectrometer is another type of existing mass spectrometers. It uses a two or three dimensional electrostatic trapping field to trap ions to be analyzed and an excitation pulse to excite the ions into coherent motions so that they can be detected. Like in FTICR, here ion formation, ion excitation, and ion detection are done sequentially in time. Thus, it also has a disadvantage of low duty cycle.
Additionally, the existing Fourier transform mass spectrometers have a second disadvantage in that they have a poor dynamic range and are slow. The third disadvantage they have is that resolution can be degraded due to ion-ion and ion-molecule collisions because they have to keep the ions trapped for a relatively long time to get the measurement done and by other factors including field imperfections.
The disadvantages of the prior art are overcome by the present invention, which, in one aspect, is a continuous beam Fourier transform mass spectrometer that is capable of providing a mass spectrum with less dependence upon ion collisions and can be operated in a 100% duty cycle. The present invention, in analyzing ions trapped in a confinement structure, utilizes a continuous excitation signal, instead of an excitation pulse used in the prior art, to the confinement structure to cause resonant motions of the ions. The signals responsive to the resonant motions of the ions can then be detected to produce a mass spectrum.
In one aspect, the present invention relates to a continuous Fourier transform mass spectrometer that includes a confinement structure having a cavity, a first opening and a second opening. The spectrometer also includes means for applying an RF voltage to the structure to form a trapping field in the cavity and means for supplying a continuous beam of ions through the first opening to the cavity to form a sample of ions with a range of masses. The sample ions are trapped in the trapping field and each ion is characterized by a mass-to-charge dependent frequency of motion. The spectrometer further includes means for continuously applying an excitation signal having a frequency spectrum and an amplitude to the trapped sample ions. The frequency spectrum of the excitation signal includes characteristic frequencies corresponding to at least one of the mass-to-charge dependent frequencies of motion of the sample ions, and the amplitude of the excitation signal is sufficient high to cause resonant motions of the ions with at least one of the characteristic frequencies of the excitation signal. The spectrometer further has means for detecting signals responsive to the resonant motions of the ions, wherein the second opening allows at least some of the sample ions to exit the cavity. Because the ions are continuously fed into the cavity and excited into resonant motions continuously by the excitation signal that can be detected continuously, the spectrometer can offer a mass spectrum with fewer ion collisions and can be operated in a 100% duty cycle.
In another aspect, the invention is a continuous beam Fourier transform mass spectrometer including a quadrupole structure having end caps and a ring electrode. The end caps and the ring electrode are spaced apart from each other thereby defining a cavity that includes a first opening and a second opening communicating with outside. The spectrometer has means for applying an RF voltage to the ring electrode to form a three-dimensional trapping field in the cavity. Furthermore, the spectrometer includes ion beam means for supplying a continuous beam of ions through the first opening to the cavity to form a sample of ions with a range of masses. The sample ions are trapped in the trapping field and each ion is characterized by a mass-to-charge dependent frequency of motion. The spectrometer further has excitation means for continuously applying an excitation signal having a frequency spectrum, which includes characteristic frequencies corresponding to at least one of the mass-to-charge dependent frequencies of motion, to at least one of the end caps to cause resonant motions of the trapped sample ions with at least one of the characteristic frequencies of the excitation signal. The ions in resonant motions are ejected away from the cavity through the second opening continuously thereby to form a current. The spectrometer has means for detecting the current and produces a mass spectrum from the detected current.
In a further aspect, the invention relates to a continuous beam Fourier transform mass spectrometer that has a quadrupole structure having a plurality of linear quadrupole rods. The linear quadrupole rods are spaced parallel and apart from each other thereby defining a bore extending axially between the ends of the structure. The bore has a longitudinal axis. The spectrometer has means for applying RF voltage signals selectively to the rods so that RF voltage signals applied to adjacent rods are 180xc2x0 out-of -phase and RF voltage signals applied to opposing rods are in-phase thereby to form a two-dimensional trapping field radially in the bore. The spectrometer also has means for supplying a continuous beam of ions through one end of the structure to the bore along the longitudinal axis to form a sample of ions with a range of masses. The sample ions are trapped by the trapping field radially and transmitted through the bore axially, with each ion characterized by a mass-to-charge dependent frequency of motion. The spectrometer further includes excitation means for continuously applying an excitation signal having a frequency spectrum, which includes characteristic frequencies corresponding to at least one of the mass-to-charge dependent frequencies of motion, to a pair of opposing rods to cause resonant motions of the trapped sample ions with at least one of the characteristic frequencies of the excitation signal. The ions in resonant motions move in expanded radii of motion. The spectrometer has means for detecting the ions in resonant motions and produces a mass spectrum accordingly.
The present invention in yet another aspect relates to a continuous beam Fourier transform mass spectrometer that includes a cell structure having a first pair and second pair of opposing plates and a bore extending between the ends of the structure. The bore has a longitudinal axis. The spectrometer has means for applying a uniform magnetic field in the bore. The magnetic field has a direction along the longitudinal axis thereby to form a two-dimensional trapping field radially in the bore. The spectrometer also has ion beam means for supplying a continuous beam of ions through one end of the structure to the bore along the longitudinal axis to form a sample of ions with a range of masses. The sample ions are trapped radially in the bore and each ion is characterized by a mass-to-charge dependent frequency of motion. The spectrometer further includes excitation means for continuously applying an excitation signal having a spectrum of frequency and an amplitude to the first pair of opposing plates to cause resonant motions of the trapped sample ions with at least one of the characteristic frequencies of the excitation signal. The ions in resonant motions move in expanded radii of motion thereby to approach the second pair of the opposing plates and induce an image current therein. The spectrometer has means for detecting the image current and produces a mass spectrum accordingly.
Yet another aspect of the present invention is related to a method of mass analyzing ions trapped in a confinement structure, wherein the confinement structure has a cavity. A trapping field is formed in the cavity and a continuous beam of ions is supplied therein to form a sample of ions with a range of masses. The sample ions are trapped in the trapping field and each ion is characterized by a mass-to-charge dependent frequency of motion. An excitation signal having a frequency spectrum and an amplitude is continuously applied to the trapped sample ions, wherein the frequency spectrum of the excitation signal includes characteristic frequencies corresponding to at least one of the mass-to-charge dependent frequencies of motion of the sample ions, and the amplitude of the excitation signal is sufficiently high to cause resonant motions of the ions with at least one of the characteristic frequencies of the excitation signal. Signals responsive to the resonant motions of the ions are then detected to produce a mass spectrum accordingly.
In yet another aspect, the present invention relates to a method of mass analyzing ions trapped in a cell structure, wherein the cell structure has a bore, the bore having a longitudinal axis and extending axially between a first and a second openings. A magnetic field is applied to the cell structure to form a trapping field in the bore. The magnetic field has a direction parallel to the longitudinal axis. A continuous beam of ions is supplied through the first opening to the bore to form a sample of ions with a range of masses. The sample ions are trapped radially in the trapping field and each ion is characterized by a mass-to-charge dependent frequency of motion. An excitation signal having a frequency spectrum and an amplitude is continuously applied to the trapped sample ions, wherein the frequency spectrum of the excitation signal includes characteristic frequencies corresponding to at least one of the mass-to-charge dependent frequencies of motion of the sample ions, and the amplitude of the excitation signal is sufficient high to cause resonant motions of the ions with at least one of the characteristic frequencies of the excitation signal. The signals responsive to the resonant motions of the ions are detected to produce a mass spectrum accordingly.
These and other aspects of the invention will become apparent from the following description of the preferred embodiments taken in conjunction with the following drawings. As would be obvious to one skilled in the art, many variations and modifications of the invention may be effected without departing from the spirit and scope of the novel concepts of the disclosure.